Proliferation of digital High Definition Television and of Digital Home Theater increases the demand for high quality format converters. In traditional processes, additional pixels are replicated from original ones and added to the input image to perform an up-conversion and conversely, pixels are simply dropped to perform a down-conversion. These methods are simple and easy to implement in hardware, but they introduce undesired artifacts like aliasing effects which are caused by a resampling method that does not respect the Nyquist criterion. Another approach utilizes a bilinear interpolation method to interpolate pixels. This method reduces substantially aliasing effects, but also reduces edge and texture sharpness.
In U.S. Pat. No. 5,928,313, Jul. 27, 1999, a simple FIR polyphase is proposed to resize images. This structure has some drawbacks. Since the FIR filter involved has only a predetermined set of coefficients, one cannot utilize a generic conversion ratio without introducing distortions like “horizontal and/or vertical stripes” caused by inappropriate selection of the phase coefficients. Also in U.S. Pat. No. 5,574,572, Nov. 12, 1996, a linear interpolation technique is used, where only two (2) input pixel values are weightily averaged to form the output pixel value. This results in an output image with blurred edges caused by the averaging effect.
A more elaborate method, disclosed in U.S. Pat. No. 5,661,824 Aug. 26, 1997, uses an approach based on edge map detection from which other additional pixels are generated, each having a level that does not cross boundary values set by the edge map. This approach can provide high quality scaled images with crisp edges, but it utilizes an iterative stage and thus is computationally costly for hardware implementation. In U.S. Pat. No. 5,796,879 Aug. 18, 1996, a method called area-based interpolation uses the CCD modelisation to recreate high resolution pixels. The system assimilates a pixel intensity level to the integral of a curve over a small area. The process of up sampling is equivalent to subdividing the original pixel in finer spacing pixels whose intensity levels are obtained by integrating the same curve over a smaller area. This concept can produce high-resolution-like results, but has many drawbacks for real time scaling, such as noise sensitivity and hardware cost.
In view of these considerations, it becomes important to have a scaling process that can produce artifact-free results and can remain effective to implement in hardware.